ci: use `dist: bionic` & apply `opensuse-leap-15` SCP error workaround

* Automated using https://github.com/myii/ssf-formula/pull/22

.travis.yml

 # -*- coding: utf-8 -*-
 # vim: ft=yaml
 ---
+dist: bionic
 stages:
   - test
   - lint
 sudo: required
 cache: bundler
 language: ruby
-dist: xenial
 
 services:
   - docker

kitchen.yml

         - curl -o bootstrap-salt.sh -L https://bootstrap.saltstack.com
         - sh bootstrap-salt.sh -XdPbfrq -x python3 git develop
       run_command: /usr/lib/systemd/systemd
+    # Workaround to avoid intermittent failures on `opensuse-leap-15`:
+    # => SCP did not finish successfully (255):  (Net::SCP::Error)
+    transport:
+      max_ssh_sessions: 1
   - name: amazonlinux-2-develop-py2
     driver:
       image: netmanagers/salt-develop-py2:amazonlinux-2
     driver:
       image: netmanagers/salt-2019.2-py3:opensuse-leap-15
       run_command: /usr/lib/systemd/systemd
+    # Workaround to avoid intermittent failures on `opensuse-leap-15`:
+    # => SCP did not finish successfully (255):  (Net::SCP::Error)
+    transport:
+      max_ssh_sessions: 1
   - name: amazonlinux-2-2019-2-py2
     driver:
       image: netmanagers/salt-2019.2-py2:amazonlinux-2
     driver:
       image: netmanagers/salt-2018.3-py2:opensuse-leap-15
       run_command: /usr/lib/systemd/systemd
+    # Workaround to avoid intermittent failures on `opensuse-leap-15`:
+    # => SCP did not finish successfully (255):  (Net::SCP::Error)
+    transport:
+      max_ssh_sessions: 1
   - name: amazonlinux-2-2018-3-py2
     driver:
       image: netmanagers/salt-2018.3-py2:amazonlinux-2
     driver:
       image: netmanagers/salt-2017.7-py2:opensuse-leap-15
       run_command: /usr/lib/systemd/systemd
+    # Workaround to avoid intermittent failures on `opensuse-leap-15`:
+    # => SCP did not finish successfully (255):  (Net::SCP::Error)
+    transport:
+      max_ssh_sessions: 1
   - name: amazonlinux-2-2017-7-py2
     driver:
       image: netmanagers/salt-2017.7-py2:amazonlinux-2